Automated warehouses and automated storage and retrieval systems (AS/RS) can reduce costs, pilferage, and damage in storing goods in part because far fewer workers are needed for otherwise similar operations. In typical AS/RS systems, a crane-like apparatus attached to the ceiling and floor of the warehouse is used to transport and position loads. Some systems have developed motorized carts which are used to transport, access, and store pallets of material in multi-story railed structures. For example, in a warehouse, a product on a pallet may be lifted and retracted into a loading elevator shaft by a forklift-like attachment (FLA). The load is transported to a desired level of the railway, removed from the elevator, and placed on a temporary pallet support near the elevator. Next, an aisle cart corresponding to the level where the load was transported moves underneath the load. A lifting row cart matched to the aisle cart is positioned on top of the aisle cart and actuates a lifting mechanism to remove the load from the temporary pallet support. The aisle cart then transports the load and row cart down the aisle to a designated row, where the row cart separates from the aisle cart while carrying the load down a row railway to the final destination of the load. When a load is retrieved from the storage structure, the process is reversed. These actions are typically all automated by a control center at the warehouse.
Some versions of these carts communicate with the control center via a tethered cable connection. The tethered cables act as a conduit for control signals being sent to the carts but also add substantial cost, weight, and maintenance burden.
Efficiently using storage space in a warehouse requires accurate tracking of the movements of the carts as they move loads and tightly pack them in the rows. Accurate positioning also improves safety for the products stored and the system itself, since incorrect positioning may lead to collisions of devices or products. The position of a row cart is typically determined by using an encoder that counts the rotations of a motor as the cart moves across the railway. The system tracks the encoder counts and determines the total distance traveled using a function of the encoder counts and a wheel radius of the cart. This method may produce error, however, since loading a cart may change the diameter of the wheels. For example, some carts use urethane wheels to improve grip, but they have a tendency to deform under a substantial load. The encoder is acceptably accurate in determining the distance traveled for an unloaded lifting cart since the wheels are normally shaped, but when the wheels are loaded by a heavy pallet and stored material, they may deform, reducing the wheel radius between the axle and the railway and thereby introducing error.
A loaded cart may have a wheel radius from an axle to the rail that is slightly smaller than an unloaded cart, so the distance traveled by one rotation of the wheel is slightly different. If the cart is instructed to deliver a load 10 meters down a railway from an aisle cart by moving 5,000 encoder counts it may actually end up slightly farther than 10 meters after 5,000 counts due to wheel deformation. As a result, the load may not be positioned at the precise location expected by the controller, and when the next load is positioned in that row, a collision may occur. Afterward, when the row cart is instructed to return 5,000 counts to its home position on the aisle cart, the row cart may end up 0.1 meters out of position since on the return trip the wheels are relatively unloaded.
Furthermore, the error introduced by wheel deformation may be inconsistent due to variations in the weight of palletized loads or temperature. The deformation may also vary from wheel to wheel on the same cart due to uneven palletized load weight distribution.
Steel wheeled row carts have much less deflection than those with urethane wheels, but they also have a tendency to slip on the rails, leading to problems of their own. Carts with steel wheels typically must undesirably move more slowly and inefficiently to avoid introducing encoder error.